As The Walking Dead fans, we were first introduced to the zombie virus in episode 1.06 "T.S. 19" by Dr. Edwin Jenner.
We, the fans of The Walking Dead, know we will never see exactly how the Zombie Virus is created or how it became uncontained (The CDC's Wildfire). However, we can give a differential diagnosis to the disease and speculate on the disease based on how it would affects the body.
According to E. Paul Zehr and Stephanie Norman in the book "The Walking Dead Psychology," edited by Travis Langley, the zombie condition is referred to as Walker Dysfunction Disorder (WDD). Some symptoms to WDD is reduced posture and balance, slowed movement (speed), resistance to movement (rigidity) which could be caused by Parkinson's disease and could indicate that it is possible the Basal ganglia is affected. Other symptoms of WDD include slowed walking speed and speech problems which could indicate stroke like effects on the brainstem and frontal cortex. WDD is a latent, meaning it only expresses itself after death of the host.
WDD is probably spread similarly to the influenza virus, affects the nervous system in a similar fashion to meningitis, and the host reacts and displays the virus similar to rabies hosts.
The above article is a mixture of a summary of chapter three of "The Walking Dead Psychology" and my personal theories. I highly recommend getting the book; it is a fantastic read!
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